This invention relates generally to electromechanical robots for use in vacuum chamber or other non-atmospheric environments and, more particularly, to vacuum chamber robots using magnetic couplers for transmitting mechanical torque through a vacuum chamber wall without the need for vacuum seals. Robots of various types are often used to move workpieces within a vacuum environment. For example, in semiconductor fabrication, substrates or wafers may need to be moved in and out of processing chambers in a vacuum or near-vacuum environment.
There are two related problems in the design of vacuum chamber robots capable of angular and radial motion. In a typical processing system, the vacuum chamber is centrally located with respect to surrounding processing chambers, and the robot functions to move substrates or other workpieces from one processing chamber to another. One problem with this arrangement is that the vacuum chamber may have to be relatively large to accommodate a large substrate as it is withdrawn from a processing chamber, rotated to a new position, and inserted into another processing chamber. A design goal is to minimize the size of the vacuum chamber, since this results in a lower manufacturing cost, a processing system of smaller volume, and a reduced likelihood of contamination. Reduction of the size of the vacuum chamber may be difficult to achieve if there are many surrounding processing chambers that contribute to the size of the system, but volume reduction is usually an important goal when the number of processing chambers is relatively small.
One way to minimize the vacuum chamber volume is to provide a robot capable of positioning a substrate over the robot's center of rotation. Then the robot and substrate can be rotated in a chamber of the smallest possible size. Providing a robot that can be moved to an over-center position requires a robot drive mechanism that is coupled into the chamber from beneath the chamber. The present invention is concerned with magnetic couplers for transmitting the angular movement of coaxial shafts into the vacuum chamber.
Drive motors for a vacuum chamber robot are preferably mounted outside the chamber, and angular movement must be transmitted through the vacuum chamber wall. Designing the chamber wall to accommodate one or more rotatable shafts that pass through it is an unsatisfactory approach because seals around the drive shafts eventually fail. In the case of a robot providing movement in two dimensions, two independent drive motors are required, further complicating the problem. In some robots, such as the one described in U.S. Pat. No. 4,951,601 to Maydan et al., it is desirable to transmit the two drive torques into the vacuum chamber coaxially. As discussed above, a coaxial drive mechanism is also needed if it desired to move the robot to a compact, over-center position.
Magnetic torque couplers have been used for many purposes in the past. Basically, a magnetic torque coupler consists of two sets of magnets, usually permanent magnets, mounted on separate, but often coaxial, shafts and separated by a vacuum barrier. The sets of magnets are coupled together magnetically, such that rotation of one of the shafts causes synchronous rotation of the other.
Although this concept is a simple one, there are significant difficulties in implementing it in a practical context, especially when providing coupling for two coaxial drives. A fundamental requirement is that the mechanical coupling has to be strong or "stiff." Rotation of one set of magnets has to result in practically synchronous rotation of the other, with minimal lag in response. Loose magnetic coupling results in robot positioning inaccuracies or, at best, in significant positioning delays. If two couplers are used to couple two drive shafts into the vacuum chamber, the couplers must typically be spatially separated to minimize unwanted cross-coupling between them.
Accordingly, there is still need for improvement in the field of vacuum chamber robots. The present invention addresses this need.